Treatment of bayer process digester slurry



United States Patent 3,469,935 TREATMENT OF BAYER PROCESS DIGESTER SLURRY Kizhakke Govind Hrishikesan, Little Rock, and James Franklin Kane, Benton, Ark., Edward Bruce Teas, Corpus Christi, Tex., and William Haddon Brown, Little Rock, Ark, assignors to Reynolds Metals Company, Richmond Va., a corporation of Delaware No Drawing. Filed May 18, 1965, Ser. No. 456,817

- Int. Cl. C01f 7/06, 7/34 US. Cl. 23-52 6 Claims ABSTRACT OF THE DISCLOSURE Dissolved zinc is removed from an aqueous alkaline slurry obtained by digestion of a zinc-containing bauxite with caustic alkali, by adding to the slurry an alkali metal sulfide to cause precipitation of the zinc as zinc sulfide, and holding the slurry in a quiescent state for 15 to 45 minutes to allow the zinc sulfide to collect on the insoluble bauxite residue. The resulting clarified digestion liquor may be further treated by filtering through a bed of red mud. The alkali metal sulfide may be derived from carbothermic reduction of a mixture of sodium sulfate, sodium carbonate and sodium aluminate, known as salting out product, formed as a by-product in the processing of sodium aluminate to recover alumina hydrate.

This invention relates to an improved method for the removal of zinc and other metal impurities from alkali metal aluminate solutions. More particularly, the invention concerns the removal of zinc from Bayer process liquors by precipitation with an alkali metal sulfide.

The novel method of the invention is adapted to the processing of zinc-containing aluminous ores, and particularly ores of the bauxite type. The term bauxite is generally used to designate a natural aggregate of aluminumbearing minerals, more or less impure, in which the aluminum occurs largely as hydrated oxides.

The most commonly used method for extracting alumina from bauxite is the Bayer process. In accordance with this process, the ground bauxite is digested with an aqueous solution of an alkali, such as, for example, caustic soda, or with a mixture of caustic soda and sodium carbonate. There is obtained a slurry which comprises a suspension of the constituents of the bauxite other than alumina and which are insoluble in the alkali liquor. The liquor contains the alumina dissolved in the form of a supersaturated sodium aluminate solution. The constituents of the bauxite which remained unattacked during the alkali digestion from an insoluble residue, known as red mud. For recovery of the alumina, the sodium aluminate solution or Bayer liquor is separated from the red mud by filtration or sedimentation or a combination of both. The clarified liquor comprises an unstable solution of alumina from which most of the dissolved alumina is precipitated by seeding with alumina hydrate. The precipitated alumina hydrate is then separated from the liquor by sedimentation and filtration, washed, and calcined at high temperature to form alumina.

Purity specifications for metallic aluminum currently limit the zinc content to 0.03% or less. In order to meet this specification, it is necessary for the calcined alumina from the Bayer process to contain no more than 0.02% zinc oxide. Correspondingly, the Bayer liquor from which the alumina is produced should have a maximum zinc content of no more than 12 parts per million, or 0.012 grams per kilogram.

'Carribean bauxite ores have come into increased use by the American aluminum industry, among which are 3,469,935 Patented Sept. 30, 1969 Jamaican and Haitian bauxites which are relatively high in zinc. Owing to the solubility of zinc oxide in caustic alkalis, a substantial proportion of this zinc finds its way into the Bayer process liquors. Thus, for example, on Bayer digestion of Haitian bauxite at 435 F., the ZnO content of the pregnant liquor was found to be as high as 0.024 g./kg. In another case, on Bayer digestion of a Jamaican bauxite at 460 F. the ZnO content of the pregnant liquor was about 0.019 g./kg. In U.S. Patent 2,885,261, it is proposed to reduce the zinc content of Bayer liquor to a low level by precipitation of zinc sulfide, by the addition of sodium sulfide to the clarified sodium aluminate liquor after removal of the insoluble residue (red mud). The patent indicates that the addition of sodium sulfide to the unclarified digester eflluent was less eifective for the removal of the zinc.

In accordance with the present invention, it has been found, surprisingly and unexpectedly, that a reduction in the Zinc content of the clarified Bayer liquor to as little as 10 p.p.m., or 0.010 g./kg. can be successfully achieved by the addition of an alkali metal sulfide to the unclarified bauxite digester slurry and then holding the slurry in a quiescent state for a period of about 15 to about 45 minutes, preferably about 30 minutes. In this way, the red mud, or insoluble bauxite residues, acts. as a collector for the precipitated zinc sulfide, greatly facilitating its effective removal.

While not wishing to be bound by any particular theory, it is believed that the problem of zinc removal is essentially one of collecting of the colloidal zinc sulfide, rather than of the precipitation itself. The colloidal character of precipitated zinc sulfide is well known, and X-ray diffraction studies of precipitated zinc sulfide have shown the presence of extremely small crystallites, of colloidal dimensions. This is confirmed by the finding that the zinc sulfide is capable of being adsorbed by the red mud. Any considerable agitation will cause the zinc sulfide to desorb from the red mud, and an adsorption or growth time after the addition of the precipitant sodium sulfide is needed to promote the formation and collection of the zinc sulfide.

The effectiveness of red mud as a collector for the zinc sulfide is shown by the fact that decantation washing of the settled mud from treated slurry did not cause zinc sulfide to desorb from the mud and return to the liquor, in cases where the proper holding period was employed.

Although zinc reduction can be achieved, in accordance with the invention, by addition of the stoichiometric quan tity of alkali metal sulfide, an excess over the stoichiometric quantity is preferred in order to bring about better collection of zinc sulfide on the red mud. Thus, the alkali metal sulfide, such as sodium or potassium sulfide, which is employed as the precipitation agent, is added to the digester slurry in an amount between about 0.14 and about 0.22 g./kg., preferably about 0.18 g./kg.

The temperature of digested slurry after flashing down to atmospheric pressure is about 220 F., and its has been found best to add the sodium sulfide to the digested slurry at a temperature of 220 F. or less. If the digested slurry is much hotter than 220 F. when the Na S is added, as would be the case if addition were done in the flash tanks, equivalent zinc removal is obtained only by increasing the dosage level of Na s to about 0.36 g./kg.

It is common practice to add starch as a settling agent in the processing of Bayer liquors. In the practice of the present invention, it has been found that more complete removal of zinc is achieved by adding the alkali metal sulfide to the slurry prior to the addition of the starch.

It has been found, in accordance with the invention, that the alkali metal sulfide employed as indicated also aids in the removal of other heavy metals which are capable of forming sulfides insoluble in caustic alkali, such as lead or copper. Thus, a reduction in lead content from 1.5 p.p.m. down to an amount not readily detectable is concurrently achieved.

The treatment of a digester discharge slurry with 0.18 g./kg. of sodium sulfide, in accordance with the invention, can reduce the zinc oxide content of the Bayer liquor from 0.024 to 0.010 g./kg. In accordance with another aspect of the invention, it has been found that the zinc content can be further reduced, to a level as low as 0.006 g./kg. by passing the liquor through a polishing filter, using as a filter medium a regular red mud filter cake obtained from the conventional clarification of Bayer liquor. This procedure also serves to reduce the holding time for the zinc sulfide precipitation using the red mud as the collector.

In accordance with another aspect of the invention, it has been found that there can be advantageously employed as the alkali metal sulfide precipitant for the zinc, sodium sulfide obtained by the carbothermic reduction of the sodium sulfate content of the salting out product (SOP) formed as a by-product in the evaporative processing of Bayer liquor. The salting out product is a mixture of sodium sulfate, sodium carbonate, and sodium aluminate. A typical analysis of SOP shows a composition:

Ingredient: Percent Total Na O 51.85 N21 CO NaOH 15.60

A1 8.32 S0 10.21 Cl 0.65 P 0 0.017 F 0.09 Sodium organic salts as Na CO 7.10

The Na SO in the SOP cake is carbothermically reduced to Na s to the extent of about 86% by heating a mixture of SOP cake with between about and about 30% by weight of carbon (coke breeze) for a period of about 1 hour, at a temperature between about 1380 F. and about 1740 F., preferably about 1560 F.

It was found, surprisingly and unexpectedly, that the reduced SOP cake when slurried in water is about 40% more effective in removing zinc oxide from Bayer liquor than pure sodium sulfide. Thus, by employing pure sodium sulfide in a concentration of 0.18 g./kg. the zinc oxide level in Haitian bauxite digester slurry was reduced to 0.010 g./kg., whereas employing the same concentration of sodium sulfide in the form of reduced salting out product, the zinc oxide level was reduced to 0.006 g./kg., without the benefit of red mud cake filtration. This additional reduction is believed to be attributable to the beneficial presence of undissolved solids in the SOP reduction product.

The following examples illustrate the practice of the invention, but are not to be regarded as limiting:

Example 1 Several Bayer digestions were carried out at 435 F. for 30 minutes using Haitian bauxite and caustic soda solution having the following analyses.

Haitian bauxite: Percent A1 0 45.86 F8203 sio 2.22 ZnO 0.031 Loss on Ignition 23.06

Caustic soda solution: G./kg. NaOH as N-a CO 182.1 Na CO 44.1 A1 0 60.2 ZnO 0.002

The slurries from the above digestions contained 145 Z in clarified liquor after filtration through red mud cake, gJkg.

Dosage oi Nags, g./kg.

Holding Z110 in clarified time, min. liquor, g./kg.

The above results clearly demonstrated that at a Na S dosage level of 0.18 g./kg., the ZnO level in the digested liquor was reduced from 0.024 g./kg. to 0.010 g./kg. A further reduction to 0.006 g./kg. ZnO was achieved by filtration of the liquor through a red mud cake.

Example 2 A salting out product from Bayer liquor, composed of Na CO Na SO and NaAlO analyzing 10.21% S0 was heated with 20% of its weight of coke breeze at 1560 F. for a period of 1 hour. The reduction of Na SO to Na s was about 86%. The reduced SOP cake was slurried in water to form an aqueous solution of sodium sulfide, the ratio of reduced SOP cake to water being about 46 to 52 parts by weight. 9.4 parts of NaOH were added to stabilize the alumina content to furnish a slurry containing about 2.5% Na S. This slurry was added to the Haitian bauxite digester slurry obtained in Example 1 in an amount sufiicient to furnish a concentration of 0.18 g./kg./Na S in the digester slurry. The mixture was held for 30 minutes to allow collection of the precipitated Zinc sulfide on the red mud. After settling and clarification, the zinc oxide content of the clarified liquor was found to be 0.006 g./kg. Under these conditions further filtration through a red mud cake as described in Example 1 was not necessary. By filtering the clarified liquor through a cake of red mud, however, the ZnO level in the liquor was further reduced to 0.004 g./kg.

While presently preferred practices of the invention have been described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. Method for the removal of dissolved heavy metals from an aqueous alkaline slurry obtained by digestion of an aluminous ore with caustic alkali solution, said slurry containing alkali insoluble ore residue, comprising the steps of adding to said slurry at least the stoichiometric amount of an alkali metal sulfide to cause precipitation of the heavy metals as sulfides, said alkali metal sulfide being obtained by carbothermic reduction of a mixture of sodium sulfate, sodium carbonate and sodium aluminate formed as a by-product in the processing of sodium aluminate solutions to recover alumina therefrom, holding said slurry in a quiescent state for a period of about 15 to about 45 minutes to allow the metal sulfides to collect on said insoluble ore residue, and then separating the insoluble residue and metal sulfides from the digestion liquor.

2. Method for the removal of dissolved zinc from an aqueous alkaline slurry obtained by digestion of a zinc-containing aluminous ore with caustic alkali, said slurry containing alkali insoluble ore residue, comprising the steps of adding to said slurry at least the stoichiometric amount of sodium sulfide to cause precipitation of the zinc as zinc sulfide, said "sodium sulfide being obtained by carbothermic reduction of a mixture of sodium sulfate, sodium carbonate and sodium aluminate formed as a by-product in the processing of sodium aluminate solutions to recover alumina therefrom, holding said slurry in a quiescent state for a period of about 15 to about 45 minutes to allow the zinc sulfide to collect on said insoluble ore residue, and then separating the insoluble residue and Zinc sulfide from the digestion liquor.

3. The method of claim 1 in which the carbothermic reduction is performed by heating said by-product with from about 20% to about 30% by Weight of carbon at from about 1380 to about 1740 F.

4. The method of claim 2 in which the carbothermic reduction is performed by heating said by-product with from about to about by weight of carbon at from about 1380 to about 1740 F,

5. The method of claim 1 in which the aluminous ore is a Zinc-containing bauxite.

6. The method of claim 2 in which the aluminous ore is a zinc-containing bauxite.

References Cited UNITED STATES PATENTS 1,137,860 5/1915 Howard 23-52 2,612,435 9/1952 Perrin et a1. 23-52 2,885,261 5/1959 Adams et a1. 23-143 HERBERT T. CARTER, Primary Examiner U.S. Cl. X.R. 23--135, 143 

